In contrast to specular or mirror reflection in which light is reflected back only at an angle equal to that of the incident radiation, diffuse reflection provides reflective light luminance at many angles. Typical diffuse reflectors used for example as white standards for various light measuring test instruments are made of white inorganic compounds (such as barium sulfate or magnesium oxide) in the form of pressed cake or ceramic tile, all of which are expensive, stiff, and brittle. Other diffuse reflectors are (1) microvoided particle-filled articles that depend on a difference in index of refraction of the particles, the surrounding matrix and optional air-filled voids created from stretching and (2) microporous materials made from a sintered polytetrafluoroethylene suspension.
Another useful technology for producing microporous films is thermally induced phase separation (TIPS). This technology has been employed in the preparation of microporous materials wherein thermoplastic polymer and a diluent are separated by a liquid-liquid phase separation as described in U.S. Pat. Nos. 4,247,498 and 4,867,881. A solid-liquid phase separation process has been described in U.S. Pat. No. 4,539,256. The use of nucleating agents incorporated in the microporous material is also described as an improvement in the solid-liquid phase separation method, U.S. Pat. No. 4,726,989.
Effective but inexpensive diffuse reflective articles are still needed for the many diverse light management applications that are being developed.